The present invention concerns the hermetic enclosure of hybrid circuits.
The problem of eliminating humidity in electronic devices, or at least restricting it, is a difficult one. The most common contaminating agent--and the one with the most damaging effect on the reliability of a discrete component or an integrated circuit is water from the surrounding environment. Humidity is likely to corrode the circuit contacts, an effect amplified by dissolution of ions. This effect is very serious on integrated circuits, and increases with the scale of integration, i.e. as the connections become smaller. One solution to this problem is to enclose the circuit in an atmosphere of inert gas, such as nitrogen, which must not leak away.
For high-performance applications in severe environments, such as military or space missions, electronic units comprising a base of insulating material and a metallic can are used. The can is hermetically sealed to the base, generally after degassing. The base of the unit constitutes the substrate of the circuit, for example of a multilayer hybrid circuit. It may be in ceramic. The metal can is formed by a frame topped with a cap. The frame may be a simple cylinder, or may be divided internally into separate sections.
The materials to be assembled to obtain such units have different characteristics. The substrate is of ceramic and supports a multilayer type circuit formed by serigraphy. The metal frame, on the other hand, has a coefficient of thermal expansion which is not perfectly matched to that of the ceramic. Moreover, they must be connected by a welded joint whose expansion coefficient is much higher than that of the materials to be assembled. To perform this welding, the metal must undergo a specific chemical or electrolytic treatment in order to ensure wettability. On the ceramic side, the multilayer circuit comprising strip conductors or tracks must be insulated from the metal part to avoid short-circuits. It is therefore necessary to apply a deposit of dielectric paste by serigraphy at the position of the metallic frame. Since the welding does not adhere to the dielectric material, a conducting metallic ink must be deposited on this dielectric material.
For small dimensions, mechanical stresses cause no fracturing at the interface of the assembly and sealing at the various interfaces remains satisfactory after extensive thermal cycling. However, at sizes greater than a few
square centimeters, the interface between the dielectric and the metal is too fragile.
Studies performed by the Applicant have enabled significant reduction of the problem of the airtightness of such assemblies.